1. Field of the Invention
The present invention relates to a fuser for use in an image-forming apparatus which performs an electrophotographic type image-forming process such as a printer, facsimile, copier, or complex machine including these functions and an image-forming apparatus using the fuser. In particular, the present invention relates to a fuser which fuses a toner image on a surface of a recording medium by means of heat and pressure and an image-forming apparatus using the same.
2. Description of the Related Art
An electrophotographic type apparatus, for example, a laser printer includes a rotatable photoreceptor drum. In this laser printer, after a photoreceptor layer of the photoreceptor drum is uniformly charged, an electrostatic latent image is formed by the exposure with a laser beam from a laser scanning unit, the electrostatic latent image is developed by toners as a toner image, the toner image is transferred on a recording medium (a material to be recorded) such as a transfer sheet, and the recording medium passes through the fuser such that the toner image is fused on the recording medium.
In a conventional fuser, a heat generator such as a halogen lamp is arranged in a hollow portion of a hollow cylinder body made of aluminum, for example, along the rotation axis of the hollow cylinder, and a fusing roller is heated from the inside thereof by the radiation heat of the heat generator. A pressure roller which presses the fusing roller is provided parallel to the fusing roller. A recording medium passes through the nip portion formed between the pressure roller and the fusing roller, so that unfused toners adhered on the surface of the recording medium are melted by the heat of the fusing roller, and are fused on the surface of the recording medium by the pressure.
In such a roller heating type fuser, a temperature in a sheet non-passing portion of the fusing roller, namely, the end portions of the fusing roller is increased due to long period of continuous fusing of small size recording media. If a standard size or a large size recording medium is fused in the temperature-increased condition in the end portions, hot offset may occur in a range corresponding to a small size sheet non-passing portion.
In order to prevent such hot offset, the power distribution of the heat generator is controlled such that the temperature in the end portions of the fusing roller becomes a predetermined temperature or below. It is also necessary to maintain the central portion of the fusing roller to be a required temperature or above. Therefore, the frequency of the power distribution control of the heat generator is increased, causing flicker.
As a technique which controls variations in temperature in the axial direction of the fuser, Japanese Patent Application Publication No. 2000-221835 discloses a technique which lowers the amount of heat generation in a high temperature area and increases the amount of heat generation at a low temperature area during the continuous passing of small size recording materials by using a heat generator having temperature dependency. This technique is for a belt nip method in which a heat generator is arranged in an endless film, not for a fusing roller. A nip portion is formed between the film and a pressure unit by pressing the heat generators via the film with the pressure unit from the outside of the film. Therefore, the heat conductivity of the heating member using the film is largely restricted. For this reason, such a technique can not be simply applied to a mainstream roller heating method.
Japanese Patent Application Publication No. 2008-033240 discloses a carbon lamp as a heat generator which emits infrared light, and a technique which provides a reflection member for locally limiting a heating range by the carbon lamp in a nip portion. However, the surface of the reflection member is deteriorated by the use for a long period of time, so that the light distribution is changed. For this reason, a desired temperature distribution can not be obtained, and the control of the flicker may not be maintained for a long period of time.